A photoluminescent material that emits a visible light (generally, light with a wavelength of not less than 380 nm and less than 830 nm) by UV-irradiation is used for lighting equipments, back light for liquid crystal devices and the like. As such photoluminescent material, those containing rare earth elements are often used (for example, patent documents 1-3). In addition, use of an iridium complex as a photoluminescent material has been proposed (for example, patent document 4). However, rare earth element and iridium are associated with problems in that the reservoirs are insufficient, producing countries are limited, and the cost of separation and refinement is high. Therefore, a photoluminescent material using an element other than those has been desired.
On the other hand, non-patent document 1 discloses that a tightly sealed zeolite A containing silver cluster exhibits photoluminescence. However, the document describes that maintenance of photoluminescence of the zeolite A containing silver cluster requires tight sealing with a glass ampoule, or with slide glass, cover glass and an epoxide-based adhesive. It describes that zeolite A containing silver cluster, which was left standing in air at room temperature for a long time without tight sealing, kept changing in coloration and luminescence, and finally lost luminescence. Therefore, practicalization of zeolite A containing silver cluster, described in the document, as a photoluminescent material has problems. In non-patent document 1, zeolite A containing silver is subjected to a heat treatment at 500° C. for 24 hr to allow formation of silver cluster, whereby zeolite A containing silver cluster is produced.
Patent documents 5 and 6 describe an invention that utilizes conversion of ultraviolet ray to a visible light by irradiation of invisible radiation (ultraviolet ray) to molecular sieves containing oligo atomic metal clusters. In patent documents 5 and 6, silver cluster is described as oligo atomic metal clusters, and small pore zeolites such as zeolite 3A and the like, and large pore zeolites such as faujasites X and Y and the like are described as molecular sieves. However, both patent documents 5 and 6 actually confirmed photoluminescence only in zeolite 3A (i.e., small pore zeolites) containing silver cluster. Specifically, in Example 3 of patent document 5 and Example 1 of patent document 6, zeolite 3A containing silver cluster was produced by heat-treating silver-exchanged zeolite 3A at 450° C. for 24 hr to allow formation of silver cluster, as in non-patent document 1, and the photoluminescence of the zeolite was confirmed. Therefore, patent documents 5 and 6 have not confirmed that large pore zeolites (e.g., faujasites X and Y) containing silver cluster exhibit photoluminescence. In view thereof, in the Japanese application (JP-A-2010-532911) corresponding to patent document 5, the “molecular sieves” in claim 1 at the time of filing the application were limited to small pore zeolites such as zeolite 3A, etc., and large pore zeolites such as faujasites X and Y were eliminated from the claims thereof by the Amendment of Mar. 16, 2010.